In recent years, research and development on high-density optical disks have been active, and DVD has been commercially available. Thus, the optical disks have been establishing their positions as important information recording media. Conventionally, development on DVD exclusively for playback use has been active among a variety of DVD standards. However, as DVD recorders are spread, development on recordable DVD has been progressed. Further, a recent trend proposes a Blu-ray rewritable disk (BD-RE) as a result of pursuing further high-density optical disks. Thus, the BD-RE has also been available, and used in recording of digital broadcasting contents or the like.
There are known two methods of forming a recording layer in an information recordable optical disk, namely, vapor deposition and spin coating, as disclosed in Japanese Unexamined Patent Publication No. 2003-109246, for instance. These two recording layer forming methods are described referring to FIGS. 13A and 13B.
FIGS. 13A and 13B are cross-sectional views each showing an optical disk. FIG. 13A shows an example of a BD-RE, wherein a recording layer made of a phase change material is formed by vapor deposition. FIG. 13B shows an example of a DVD-R, wherein a recording layer made of an organic-pigment-based material is formed by spin-coating.
As shown in FIG. 13A, the BD-RE is produced by forming a reflecting layer 502 on an injection-molded base member 501 by sputtering or a like technique, forming a recording layer 503 on the reflecting layer 502 by vapor deposition, and attaching a sheet member 505 to the recording layer 503 with an adhesive layer 504 being formed between the recording layer 503 and the sheet member 505. Assuming that a portion of an asperity on each layer including the base member 501, which is formed on the side of the disk where an optical pickup device for emitting laser light is arranged, is called as a “groove track”, and a portion of the asperity on each layer, which is formed on the side of the disk opposite to the side where the optical pickup device is arranged, is called as a “land track”, information is recorded on the groove track.
As shown in FIG. 13B, a base member 506 and a base member 510 each produced by injection molding are attached to each other via an adhesive layer 509 by spin-coating. Prior to the attachment of the base members 506 and 510, a recording layer 507 is formed on the base member 506 by spin-coating, and a reflecting layer 508 is formed on the recording layer 507 by sputtering or a like technique. Similarly to the disk as shown in FIG. 13A, assuming that a portion of an asperity on the base member 506, which is formed on the side of the disk where an optical pickup device for emitting laser light is arranged, is called as a “groove track”, and a portion of the asperity on the base member 506, which is formed on the side of the disk opposite to the side where the optical pickup device is arranged, is called as a “land track”, information is recorded in the groove track.
To realize high-density recording in the BD-RE as shown in FIG. 13A, laser light of a shorter wavelength than the one used for an ordinary DVD is used, and an objective lens having a high numerical aperture (NA) is used. In view of this, in the BD-RE as shown in FIG. 13A, laser light is emitted from the side of the sheet member 505 having a smaller thickness than the base member 501, in place of being emitted from the side of the base member 506, as in the DVD-R having the asperity pattern as shown in FIG. 13B.
The spin-coating technique is advantageous in shortening the time required for forming a recording layer. Applying this technique in producing a BD may result in a construction as shown in FIG. 13C. Specifically, as shown in FIG. 13C, a BD is produced by forming a reflecting layer 512 on an injection-molded base member 511 by sputtering or a like technique, forming a recording layer 513 on the reflecting layer 512 by spin-coating, and attaching a sheet member 515 to the recording layer 513, with an adhesive layer 514 being formed between the recording layer 513 and the sheet member 515. In the BD as shown in FIG. 13C, assuming that a portion of an asperity on each layer including the base member 511, which is formed on the side of the disk where an optical pickup device for emitting laser light is arranged, is called as a “groove track”, and a portion of the asperity on each layer, which is formed on the side of the disk opposite to the side where the optical pickup device is arranged, is called as a “land track”, it is desirable to record information on the land track.
In other words, since the recording layer 513 is required to have a certain thickness, the recording layer 513 is required to have a large thickness as shown in FIG. 13D if information is to be recorded on the groove track, with the result that a material cost of the disk as shown in FIG. 13D is increased, as compared with the disk as shown in FIG. 13C.
A BD-RE of a type in which information is recorded on a groove track, as shown in FIG. 13A, is well known. Under the circumstances that there are two types of disks, wherein one type of disk is such that information is recorded on a groove track, and the other type of disk is such that information is recorded on a land track, a recording/playback apparatus has difficulty in readily judging whether the disk is of a type having a configuration that information has been or is to be recorded on the land track or on the groove track, with the result that it takes a certain time to start up the recording/playback apparatus.